Bulletin 30-015.003
Copyright © 2017 Unico Inc.
Page 12
over the outlet is the most convenient instrument to
use. The Turbo-Meter will give a direct LED readout
on the KNOTS (FPM x 100) setting, when multiplied
by a simple factor gives the CFM of the outlet within
an accuracy of 10%. Refer to Technote 113 for more
information on use of the Turbo-Meter.
By measuring and totaling the CFM of all outlets and
comparing the total to the SCB readout, one can
determine whether there is gross leakage in the duct
system. If the values are more than 20% or 150 CFM
apart, inspect the duct system for leaks and repair.
Refer to Bulletin 30-039 for checking airflow for an
EC motor.
Static Pressure.
Measure the external static pressure
(see figure 13) in the supply plenum at least two feet
(0.6 m) from the unit and verify that it is within the
allowable range.
It is not necessary to measure the return duct static
pressure unless it was field fabricated. The maximum
return static pressure (including filters) should be 0.15
inches of water column (37 Pa). If it is greater than
0.15 inches of water column, add the return system
pressure drop to the supply plenum static pressure to
get the total static pressure drop.
For example:
If the supply static pressure is
measured to be 1.6 inches w.c. and the return
system pressure drop is 0.25 inches w.c., the
total static pressure drop is: 1.6 + 0.25 = 1.85.
In this case the static pressure is too high.
How to Measure Static Pressure.
Measure the supply
plenum static pressure at least 24 inches (610 mm)
from the unit, but before any tee or elbow. A distance
of between 2 and 3 feet (0.6 to 0.9 m) is best. Use an
inclined manometer capable of reading at least 2.5
inches of water column (622 Pa), such as Dwyer
Instrument’s model 109 manometer. Be sure to zero
the scale and level the manometer. A magnehelic
gauge that measures up to at least 2.5 inches of water
may also be used.
Use a metal tube, typically 1/4-inch (6 mm) diameter,
to measure the static pressure. Determine where you
want it and cut or punch a small hole in the duct. Make
the hole the same size as the metal tube to prevent
leakage. Insert the metal tube one-inch (25 mm) so that
the tip of the tube is flush to inside wall of the duct and
perpendicular to the air stream as shown in Fig. 13.
Attach the metal tube to the manometer using a rubber
hose (usually supplied with the manometer). Record
the pressure.
Note: If the tube is not perpendicular to the air
stream, the reading will be in error. You will
get a higher reading if the tube is angled
toward the air stream.
Figure 15 Measuring Plenum Static Pressure
CHARGING A COOLING SYSTEM
DO NOT VENT REFRIGERANT TO THE
ATMOSPHERE!!
It is a violation of federal law and
in some cases local ordinances. Always use a
refrigerant recovery or recycling device.
If charging a heat pump, refer to
Bulletin 30-024,
Instructions for "M" Series Modular Heat Pump
Systems
. The following procedure is only valid for
charging the system during the cooling mode.
To check for proper charge, record the refrigerant
pressures and temperatures. Check the refrigerant
charge by measuring the amount of sub-cooling (or
‘approach’ temperature for some condensing units). If
the outdoor manufacturer does not have sub-cooling or
“approach” temperature charts, then be sure that the
sub-cooling is between 8 and 5 °F (2 to 5°C). For long
refrigerant lines or when the evaporator is above the
condenser, the sub-cooling should be close to 8°F;
otherwise, aim for the low end of the range.
After the refrigerant lines and evaporator have been
carefully leak tested and evacuated, release the R410A
operating charge in the condensing unit. The system is
now ready for refrigerant charge adjustment.
Start up the system and check line voltage to assure it
is within acceptable limits for the system as dictated
by the condensing unit manufacturer. Run the system
for 20 to 30 minutes to get reasonably stabilized
conditions. Do not attempt to adjust charge with
outdoor temperature below 75°F (24 °C). An outdoor
temperature of 75 to 85°F (24 to 29 °C) is preferred.
If the system charge must be checked when the
outdoor temperature is below 80°F (26.7 °C), block the
condenser coil until the head pressure is approximately
